1. Field of the Invention
This invention relates to polycarbonate copolymers and more particularly to polycarbonate copolymers having improved critical thickness values.
2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded articles where impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
However, one deficiency of polycarbonate when used in molded articles is the low critical thickness values of the polycarbonate polymer.
It is known that polycarbonate plastics exhibit high notched Izod (ASTM test D-256) impact values. This value, however, is dependent upon the thickness of the test specimen. Typical notched Izod impact values for a 1/8" specimen are about 16 ft.-lbs. per in. These high Izod values result because specimens of 1/8" thickness are thinner than the critical thickness of the polymer and therefore upon impact a hinged or ductile break occurs. On the other hand, 1/4" specimens exhibit a clean or brittle break and give notched Izod impact values of only about 2.5 ft.-lbs. per in. The 1/4" specimens are said to be above the critical thickness of the polymer. "Critical thickness" has been defined as the thickness at which a discontinuity in Izod impact values occurs. In other words, it is the thickness at which a transition from a brittle to a ductile break or vice versa occurs. Thus a standard impact specimen of polycarbonate polymer thicker than the critical thickness exhibits brittle breaks and those thinner than the critical thickness exhibit hinged or ductile breaks. Further, a polycarbonate based on bisphenol A with a melt flow of 3 to 6 grams/10 minutes at 300.degree. C. (ASTM D1238) has a critical thickness of 225 mils.
One approach to solving the critical thickness problem has been to incorporate polyolefin polymers into the polycarbonate which has substantially improved critical thickness (See U.S. Pat. No. 3,437,631). But along with this improvement have come detrimental effects such as colorant dispersion problems because of the diversity of chemical composition of the two component system and also a lack of transparency since the polyolefin and polycarbonate are incompatible.
Thus in accordance with the invention a polycarbonate is provided which has improved critical thickness values and is highly transparent.